1. Field of the Invention
Operation of an induction motor using a three-or-more level inverter bridge operating under the control of a controller designed for two-level inverter bridges.
2. Description of the Related Art
Induction motor drives, also called AC (alternating current) drives are used to control the speed and torque of multiphase induction motors, which for a long time have been the workhorse of the industry.
Today's AC drives can be divided into two categories: low-voltage and medium-voltage. The low-voltage AC drives are widely used and cover the 0 VAC to 600 VAC range. Low-voltage AC drives are manufactured by almost five hundred companies around the world. Medium-voltage AC drives cover input line voltages above 660 VAC and up to 15,000 VAC. Only about a half dozen companies design and produce medium-voltage AC drives. High-voltage AC drives cover voltages of 15,000 VAC and higher, but are very uncommon compared to low-voltage and medium-voltage AC drives. Recently, the auto industry and some other special applications requiring low output voltage harmonics are considering the use of multilevel inverter bridges for low voltage motors. This invention addresses this case as well.
Until recently, power semiconductor switches were rated at a maximum of 1,700V, which has allowed the low voltage three-phase AC drives to use a six switch inverter bridge. Today, state-of-the-art semiconductor switches are rated at 2,500V, 3,300V, 4,500V, 6,500V and can be used in a two-level six-switch inverter bridge having up to a 2,000 VAC input. Above 2,000 VAC, the inverter bridge requires a greater number of power semiconductor switches connected in series. The most popular inverter topology for three-phase medium-voltage induction motors of up to 4,000V is a three-level twelve-switch inverter bridge.
The number of levels in an inverter bridge defines the number of direct current (DC) voltage steps that are required by the inverter bridge in order to achieve a certain voltage level in its output. Because power semiconductor switches have limited voltage capability, the total DC bus voltage of an inverter bridge is divided into a number of voltage steps, such that each voltage step can be handled by one power switch.
As illustrated in FIG. 1, in a conventional two-level AC drive, three-phase AC power (R, S, T), after passing through an optional input line reactor 80, is rectified by rectifier 10 and capacitor 20 to form a two-level DC bus. Depending on the design approach, input harmonics on the DC bus may be further reduced by a DC reactor 81. The two-level DC bus voltage is applied across the six-switch inverter bridge which produces a two-level PWM voltage output.
The six switches are divided into three branches with two switches each (30-31, 32-33, and 34-35). A controller (not shown) controls each switch via the control terminals 50-55 of each switch. The three-phase motor 90 has a phase connection derived from the middle point (71, 72, 73, respectively) between two switches of a branch, and the three branches produce three phases which collectively drive the motor.
The two-levels of the DC bus constitute a positive bus and a negative bus. The top switch of each branch is connected to the positive bus and the bottom switch is tied to the negative bus. The two switches in a branch are in series (for example, switch 30 and switch 31) and therefore can not be turned-on at the same time without causing a short-circuit. In order to prevent short-circuit, switch delay times must be taken into consideration by the controller. The top switch needs to turn-off before the bottom one turns-on, and vice-versa. Each of the switches has to be able to handle the full voltage between the positive and negative busses.
In comparison to the two-level drive, in a three-level AC drive, as illustrated in FIG. 2, the DC bus has three voltage levels (relatively labeled positive, neutral, and negative), and the inverter bridge has twelve switches 130-141. The switches 130-141 are divided into three equal branches, each branch connecting to one phase of the three-phase motor 190. Thus, each branch has four switches in series (130-133, 134-137, and 138-141), and each connection to the motor 190 is derived from a middle point 171-173.
The top two switches of each branch are connected to the positive bus and behave like one switch, but they can not be turned on or off at the same time. The switch at the very top (e.g., switch 130) is turned-on after and turned-off before the other switch (e.g., switch 131), in the top pair. The bottom two switches of each branch are connected to the negative bus. The switch at the very bottom (e.g., switch 133) has to be turned-on after, and turned-off before, the other switch of the bottom pair (e.g., switch 132). The switches are controlled by signals applied via terminals 150-161. Here again, switch delay times must be taken into consideration to prevent short circuit.
For further comparison, a single branch (i.e., phase) of three, four, and five level inverter are shown in FIGS. 3A, 3B, and 3C, respectively.
The ability to utilize multiple levels has the benefit of producing an output voltage with lower harmonic distortion, in addition to providing higher output voltages with lower voltage-rated power switches. For example, a three-level inverter has lower voltage harmonic distortion than a two-level inverter bridge.
A drawback of a three-level inverter is that while a two-level inverter bridge requires only six semiconductor power switches, a three-level inverter bridge requires twelve switches, thereby increasing costs. These costs continue to increase as additional levels are utilized: a four-level inverter requires eighteen switches and a five-level inverter requires twenty-four switches.
Further increasing costs is that as the number of levels and switches in the inverter bridge is increased, the complexity of controlling the switches also increases. The signals that drive the switches need to be carefully timed—otherwise the switches may be damaged or destroyed. This complexity increases the costs of controllers used with multiple-level inverters.
Accordingly, a cost-benefit analysis typically results in multiple-level inverters being used only when the output voltages, harmonics and power requirements exceed the capabilities of two-level inverters. A side-effect of this result is that controllers for multiple-level drives are produced in much lower volume.
Any induction motor drive has to control the motor and additionally perform a large number of interfacing tasks such as: communicating diagnostics information; receiving control input from an operator and/or a host or slave process computer; receiving commands from the drive application; performing external control functions; and/or serving as a communications gateway by interfacing different serial communications protocols. These functions are all in addition to motor control, and demand large amounts of expertise and resources to be developed. As a result of the lower volumes, controllers for multiple-level systems used with medium-and-high voltage drives are more expensive and typically offer less-or-limited interfacing capabilities than corresponding controllers for two-level systems produced for low voltage AC drives. Unfortunately, common two-level modulator signals from a low-voltage induction motor controllers are not suitable for the control of multi-level inverter bridges.
Even so, to be able to utilize existing “off-the-shelf” low voltage controllers to control multiple-level inverter bridges would shorten development cycles and speed up product availability of medium-and-high voltage AC drives. A further advantage with this approach is that because low-voltage drives are produced in large volumes, the cost of the two-level controllers is optimized and the circuitry of the controllers is of superior quality and reliability.